Ultraviolet Radiation: How It Affects Life on Earth : Feature Articles By Jeannie Allen · September 6, 2001 The sun radiates energy in a wide range of wavelengths, most of which are invisible to human eyes. The shorter the wavelength, the more energetic the radiation, and the greater the potential for harm. Ultraviolet (UV) radiation that reaches the Earths surface is in wavelengths between 290 and 400 nm (nanometers, or billionths of a meter). This is shorter than wavelengths of visible light, which are 400 to 700 nm. People and plants live with both helpful and harmful effects of ultraviolet (UV) radiation from the sun. (Photograph courtesy Jeannie Allen) UV radiation from the sun has always played important roles in our environment, and affects nearly all living organisms. Biological actions of many kinds have evolved to deal with it. Yet UV radiation at different wavelengths differs in its effects, and we have to live with the harmful effects as well as the helpful ones.

Radiation at the longer UV wavelengths of 320-400 nm, called UV-A, plays a helpful and essential role in formation of Vitamin D by the skin, and plays a harmful role in that it causes sunburn on human skin and cataracts in our eyes. The incoming radiation at shorter wavelengths, 290-320 nm, falls within the UV-B part of the electromagnetic spectrum. (UV-B includes light with wavelengths down to 280 nm, but little to no radiation below 290 nm reaches the Earth’s surface). UV-B causes damage at the molecular level to the fundamental building block of life deoxyribonucleic acid (DNA). Electromagnetic radiation exists in a range of wavelengths, which are delineated into major divisions for our convenience. Ultraviolet B radiation, harmful to living organisms, represents a small portion of the spectrum, from 290 to 320 nanometer wavelengths. (Illustration by Robert Simmon) DNA readily absorbs UV-B radiation, which commonly changes the shape of the molecule in one of several ways. The illustration below illustrates one such change in shape due to exposure to UV-B radiation.

Changes in the DNA molecule often mean that protein-building enzymes cannot read the DNA code at that point on the molecule. As a result, distorted proteins can be made, or cells can die. Ultraviolet (UV) photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent bases bond with each other, instead of across the ladder. This makes a bulge, and the distorted DNA molecule does not function properly. (Illustration by David Herring) But living cells are smart. Over millions of years of evolving in the presence of UV-B radiation, cells have developed the ability to repair DNA. A special enzyme arrives at the damage site, removes the damaged section of DNA, and replaces it with the proper components (based on information elsewhere on the DNA molecule). This makes DNA somewhat resilient to damage by UV-B. In addition to their own resiliency, living things and the cells they are made of are protected from excessive amounts of UV radiation by a chemical called ozone. A layer of ozone in the upper atmosphere absorbs UV radiation and prevents most of it from reaching the Earth.

Yet since the mid-1970s, human activities have been changing the chemistry of the atmosphere in a way that reduces the amount of ozone in the stratosphere (the layer of atmosphere ranging from about 11 to 50 km in altitude). This means that more ultraviolet radiation can pass through the atmosphere to the Earths surface, particularly at the poles and nearby regions during certain times of the year. Without the layer of ozone in the stratosphere to protect us from excessive amounts of UV-B radiation, life as we know it would not exist. Scientific concern over ozone depletion in the upper atmosphere has prompted extensive efforts to assess the potential damage to life on Earth due to increased levels of UV-B radiation. Some effects have been studied, but much remains to be learned. Ultraviolet Waves - Mission:Science Ultraviolet Waves National Aeronautics and Space Administration, Science Mission Directorate. (2010). Ultraviolet Waves. Retrieved , from Mission:Science website: Science Mission Directorate. "Ultraviolet Waves" Mission:Science. 2010. National Aeronautics and Space Administration.

ULTRAVIOLET LIGHT FROM OUR SUN Bees, along with some birds, reptiles and other insects, can see near-ultraviolet light reflecting off of plants. Bug zappers attract insects with ultraviolet light to lure them to the trap. Ultraviolet (UV) light has shorter wavelengths than visible light. Although UV waves are invisible to the human eye, some insects, such as bumblebees, can see them. This is similar to how a dog can hear the sound of a whistle just outside the hearing range of humans. The Sun is a source of the full spectrum of ultraviolet radiation, which is commonly subdivided into UV-A, UV-B, and UV-C. These are the classifications most often used in Earth sciences. UV-C rays are the most harmful and are almost completely absorbed by our atmosphere. UV-B rays are the harmful rays that cause sunburn. Exposure to UV-B rays increases the risk of DNA and other cellular damage in living organisms. Fortunately, about 95 percent UV-B rays are absorbed by ozone in the Earth's atmosphere.

Credit: Image is courtesy of: NASA/SDO/AIA Scientists studying astronomical objects commonly refer to different subdivisions of ultraviolet radiation: near ultraviolet (NUV), middle ultraviolet (MUV), far ultraviolet (FUV), and extreme ultraviolet (EUV). NASA's SDO spacecraft captured the image below in multiple wavelengths of extreme ultraviolet (EUV) radiation. The false-color composite reveals different gas temperatures. Reds are relatively cool (about 60,000 Celsius) while blues and greens are hotter (greater than one million Celsius). NASA's Solar Dynamics Observatory (SDO) spacecraft captured this view of a dense loop of plasma erupting on the Sun's surface—a solar prominence. The plasma is seen flowing along a magnetic field. Credit: NASA ozonewatch.gsfc.nasa.gov DISCOVERY OF ULTRAVIOLET Johann Ritter's experiment was designed to expose photographic paper to light just beyond the visible spectrum and prove the existence of light beyond violet—ultraviolet light. Credit: Troy Benesch In 1801, Johann Ritter conducted an experiment to investigate the existence of energy beyond the violet end of the visible spectrum.

Knowing that photographic paper would turn black more rapidly in blue light than in red light, he exposed the paper to light beyond violet. Sure enough, the paper turned black, proving the existence of ultraviolet light. ULTRAVIOLET ASTRONOMY Since the Earth's atmosphere absorbs much of the high-energy ultraviolet radiation, scientists use data from satellites positioned above the atmosphere, in orbit around the Earth, to sense UV radiation coming from our Sun and other astronomical objects. Scientists can study the formation of stars in ultraviolet since young stars shine most of their light at these wavelengths. This image from NASA's Galaxy Evolution Explorer (GALEX) spacecraft reveals new young stars in the spiral arms of galaxy M81. Credit: NASA/JPL-Caltech The image to the right shows three different galaxies taken in visible light (bottom three images) and ultraviolet light (top row) taken by NASA's Ultraviolet Imaging Telescope (UIT) on the Astro-2 mission. The difference in how the galaxies appear is due to which type of stars shine brightest in the optical and ultraviolet wavelengths. Ultraviolet images of galaxies show mainly clouds of gas containing newly formed stars that are many times more massive than the Sun and glow strongly in ultraviolet light.

In contrast, visible light images of galaxies show mostly the yellow and red light of older stars. By comparing these types of data, astronomers can learn about the structure and evolution of galaxies. THE OZONE "HOLE" Chemical processes in the upper atmosphere can affect the amount of atmospheric ozone that shields life at the surface from most of the Sun's harmful UV radiation. Each year, a "hole" of thinning atmospheric ozone expands over Antarctica, sometimes extending over populated areas of South America and exposing them to increased levels of harmful UV rays. The Dutch Ozone Monitoring Instrument (OMI) onboard NASA's Aura satellite measures amounts of trace gases important to ozone chemistry and air quality. The image above shows the amount of atmospheric ozone in Dobson Units—the common unit for measuring ozone concentration. These data enable scientists to estimate the amount of UV radiation reaching the Earth's surface and forecast high-UV-index days for public health awareness. ULTRAVIOLET LIGHT FROM STARS The Lyman-Alpha Mapping Project (LAMP) onboard the Lunar Reconnaissance Orbiter can peer into permanently shaded craters on the moon by sensing the faint reflections of UV light coming from distant stars.

Credit: Ernest Wright LRO/LAMP AURORAE Aurorae are caused by high-energy waves that travel along a planet's magnetic poles, where they excite atmospheric gases and cause them to glow. Photons in this high-energy radiation bump into atoms of gases in the atmosphere causing electrons in the atoms to excite, or move to the atom's upper shells. When the electrons move back down to a lower shell, the energy is released as light, and the atom returns to a relaxed state. The color of this light can reveal what type of atom was excited. Green light indicates oxygen at lower altitudes. Red light can be from oxygen molecules at a higher altitude or from nitrogen. On Earth, aurorae around the north pole are called the Northern Lights. JUPITER'S AURORA The Hubble Space Telescope captured this image of Jupiter's aurora in ultraviolet wrapping around Jupiter's north pole like a lasso. Credit: John Clarke (University of Michigan) and NASA This unusual false-color image shows how the Earth glows in ultraviolet (UV) light. The Far UV Camera/Spectrograph deployed and left on the Moon by the crew of Apollo 16 captured this image.

The part of the Earth facing the Sun reflects much UV light and bands of UV emission are also apparent on the side facing away from the Sun. These bands are the result of aurora caused by charged particles given off by the Sun. They spiral towards the Earth along Earth's magnetic field lines. Ultraviolet (UV) / Ozone Frequently Asked Questions Bureau Home > UV and Sun Protection services > Ultraviolet (UV) / Ozone Frequently Asked Questions Ultraviolet (UV) / Ozone Frequently Asked Questions This page contains answers to frequently asked questions regarding UV radiation and the ozone layer; scroll down the page to view all the FAQs. Ultraviolet radiation What is ultraviolet (UV) radiation? Ultraviolet radiation is the process by which shorter wavelength electromagnetic (radiant) energy is transferred from one place to another, but typically from the sun through the Earths atmosphere to the Earths surface; less typically from a tanning bed lamp to a persons skin. Radiant energy (measured in joules) or irradiance (the radiant power measured in watts), is classified according to wavelength measured in nanometres (one nm is a millionth of a millimetre). The shorter the wavelength, the more energetic.

In order of decreasing energy, the principal forms of radiant energy are gamma rays, X rays, UV (ultraviolet), visible (or light), infrared, microwaves, and radio waves. There are three categories of UV: UV-A, between 320 and 400 nm UV-B, between 280 and 320 nm UV-C, between 200 and 280 nm How harmful is UV radiation? Generally, the shorter the wave-length, the more biologically damaging UV radiation can be if it reaches the Earth in sufficient quantity. UV-A reaches the Earth from the sun in the greatest quantities. Recent information suggests that UV-A can damage the skins immune system. UV-A from the sun passes right through the ozone layer to the Earths surface with little attenuation. UV-B radiation is potentially very harmful. Fortunately, most of the sun's UV-B radiation is absorbed by ozone in the stratosphere. UV-C radiation is potentially the most damaging because it is very energetic. Fortunately, all UV-C is absorbed by oxygen and ozone in the stratosphere and never reaches the Earth's surface. What factors affect the amount of UV radiation that reaches the Earth?

Although the ozone layer is the one constant defence against UV penetration, several other factors can have an effect: Ozone: Lower ozone values in the atmosphere lead to higher UV levels, and conversely, higher ozone values lead to lower UV levels. Ozone levels are affected by weather patterns in the upper atmosphere. Areas of converging air in the upper atmosphere generally have slightly higher ozone values resulting in slightly lower UV levels, and areas of diverging air in the upper atmosphere have slightly lower ozone values resulting in slightly higher UV levels. Latitude: Since the sun's UV energy impacts the Earth's surface at the most direct angle over the equator it is the most intense at this latitude. Also ozone is transported by large scale circulations in the upper atmosphere, from equatorial regions towards the polar regions, resulting in generally higher values of ozone, and therefore lower levels of UV. Time of day: On any day the greatest amount of UV reaches the Earth around midday when the sun is at its highest point. Up to 50% of daily UV radiation levels is received between 11am and 2pm. Daily changes in the angle of the sun influence the amount of UV that passes through the atmosphere.

When the sun is low in the sky, solar energy must travel a greater distance through the atmosphere and may be scattered and absorbed by water vapour and other atmospheric components. Time of year (Season): During winter months, the sun is lower in the sky than in summer, and the length of the day is shorter, hence UV is less intense. Altitude: The air is thinner and cleaner on a mountain top, therefore more UV reaches the mountain top than at lower elevations. UV levels increase by 4% for every 300 metre rise in altitude. Some ski resorts in Australia are situated near 2000m altitude and will receive UV levels up to 25% more than locations situated at sea level along a similar latitude. Cloud cover: Clouds can have a marked impact on the amount of UV that reaches the Earth's surface; generally, thick clouds reflect and absorb more UV than thin cloud cover. UV can pass through thin clouds. However, sides of clouds can also reflect UV and focus solar energy and can increases the amount received at the Earths surface in some situations.

Air pollution: Similar to the way clouds shield the Earth's surface from the suns UV, urban smog can reduce the amount of UV energy reaching the Earth by reflecting UV back towards space or absorbing UV. Land Cover: Incoming UV is reflected from most surfaces. Reflected UV can damage people, plants, and animals just as incoming UV does. Conversely, you will still receive about 40% of UV radiation levels in water at a depth of 50cm. The following table of UV reflectances is sourced from Sliney, DH. 1986. Table of UV reflectances Boat deck (wood to fibreglass) 6 to 9 Is there any link between the temperature and UV levels? There is no link between UV levels and how hot or cold the temperature is. There are differences between UV levels during Summer and Winter, but this is mostly due to the angle of the sun in the sky. UV levels generally peak around the middle of the day, whilst the temperature may still be rising towards its maximum in the afternoon. The graph below outlines this relationship.

How does the ozone layer protect us from skin damage Ozone's unique physical properties allow the ozone layer to act as our planet's sunscreen, providing an invisible filter to help protect all life forms from the sun's damaging UV. All UV-C and most incoming UV-B is absorbed by ozone and prevented from reaching the Earth's surface. Without the protective effect of ozone, life on Earth would not have evolved the way it has. How does UV-B exposure affect people? Exposure to UV-B, causes skin cancer, hastens skin aging, and can cause eye damage. The human immune system can also be weakened by exposure to UV-B. However it is important to note that UV-B has always had these effects on humans. In recent years these effects have become more prevalent because Australians are spending more time in the sunshine and are exposing more of their skin in the process. An increase in the levels of UV-B reaching the Earth as a result of ozone depletion may compound the effects that sun worshipping habits have already created. Although fair-skinned or fair-haired individuals are at highest risk for skin cancer, the risk for all skin types increases with exposure to UV-B. The effects of UV-B on the human immune system have been observed in people with all types of skin.

The incidence of skin cancer, per head of population, in Australia is ten times higher than America and more than twenty times higher than in the U.K. More than 60% of our population will be treated for skin cancer in one form or another in their lifetime. Skin cancer is very dangerous. It can be fatal if not detected early and treated correctly, usually by surgery. Over one thousand Australians die every year from melanoma and other forms of skin cancer. How does UV-B exposure affect plants and animals? Excessive UV-B inhibits the growth processes of almost all green plants. There is concern that ozone depletion may lead to a loss of plant species and reduce global food supply. Any change in the balance of plant species can have serious effects, since all life is interconnected. Plants form the basis of the food web, prevent soil erosion and water loss, and are the primary producers of oxygen and a primary sink (storage site) for carbon dioxide. UV-B causes cancer in domestic animals similar to those observed in humans. Although most animals have greater protection from UV-B because of their heavy coats and skin pigmentation, they cannot be artificially protected from UV-B on a large scale. Eyes and exposed parts of the body are most at risk.

How does snow affect UV levels? Snowcover can increase UV levels by approximately 90% in fresh snow, and by approximately 50% for old snow. The Bureau of Meteorology provides snow adjusted UV values for locations in snow covered areas between 1 June and 15 October each year. Sun protection is recommended by Cancer Council Australia whilst on the snow. How does solar UV exposure change during the year? Solar UV exposure changes throughout the year in line with the seasons. The highest exposure levels are experienced in the Summer months, and the lowest levels in the Winter months. During Spring and Autumn, solar UV exposure levels may change rapidly day to day. View our climatology maps for more detailed information. Where is ozone found in the atmosphere? About 90% of ozone is concentrated in the lower part of the stratosphere, between about 15 and 30 kilometres above the earth's surface, where it is sometimes referred to as the ozone layer. Ozone is also found naturally, at lower concentrations, in the troposphere. In the boundary layer, human made pollutants can cause the production of excess ozone. Ozone is found in the atmosphere over the entire globe although the amount varies with location and season. Why is ozone important?

Ozone is the major absorber of UVB (Ultraviolet radiant energy in the wavelength range 280-320 nanometres) in sunlight, absorbing approximately 90% of it. Many experimental studies of plants and animals, and clinical studies of humans, have shown the harmful effect of excessive exposure to UVB radiant energy. In humans these effects include increased incidence of skin cancer and cataracts. In the 1970s, scientists first raised concerns that the production and use of synthetic substances known as chloroflourocarbons (CFCs) might lead to a reduction of ozone in the stratosphere leading to an increase of UVB radiant energy. It was with the discovery of the Antarctic Ozone Hole, however, in the mid 1980s, that the issue of ozone depletion due to human activity rose to great prominence, leading to the signing of the Montreal Protocol. Ozone is also a radiatively important gas in the atmosphere which absorbs both incoming ultraviolet and outgoing infrared radiant energy, and thus has a significant impact on the earths climate. Finally, ozone remains a very useful tracer for following air movements in the stratosphere in fact this is the reason it was originally monitored.

In the troposphere, excess ozone is generated by human pollution and is harmful to human, animal and plant health. What is meant by ozone depletion? During the 1980s and early 1990s, ozone levels around the world dropped steadily, and scientists believe this reduction was primarily due to the human production of Ozone Depleting Substances (ODSs). Averaged over the globe, the size of the reduction was approximately 5%, although this amount varied considerably with location and season. However, it was also discovered in the mid 1980s that a much greater depletion of ozone had started taking place each year over Antarctica in springtime, and this phenomenon became known as the Antarctic Ozone Hole and received great publicity. During the most severe period of the Antarctic Ozone Hole (usually late September or early October) total column ozone can drop by as much as a half or even two-thirds at some locations. The Antarctic Ozone Hole has continued to form each year from the 1980s until today. What are Ozone Depleting Substances?

Ozone Depleting Substances (ODSs) are gases that contain chlorine or bromine atoms in forms able to reach the stratosphere, where they can take part in chemical reactions that destroy ozone on a large scale. The best known are the chloroflourocarbons (CFCs), formerly used extensively in refrigeration, air-conditioning, foam-blowing and as aerosol propellants, and the halons (hydrocarbon gases containing bromine), which were widely used as fire extinguishers. Production of all major ODSs is now regulated by the Montreal Protocol. Most ODSs are made by human activity but some do have natural sources as well, including methyl bromide and methyl chloride. How much has ozone been depleted over Australia? Ozone depletion in the 1980s and early 1990s was more severe in the southern hemisphere than the northern. Observations suggest that, in southern mid-latitudes (which includes Sydney, Canberra, Adelaide, Melbourne and Hobart) the amount of depletion caused by ODSs during this time was about 5%. The amount of depletion was observed to be smallest in the tropics and to increase with distance from the equator.

It is important to remember, though, that total ozone varies considerably from year to year due to natural variations, not all of which are currently fully accounted for. This means that it takes many years for clear trends to be discernible. Further long-term ozone changes can be caused by changes in transport rather than chemistry. Nonetheless, there are now tentative early indications that ozone is recovering following the success of the Montreal Protocol and its amendments. What is the Antarctic Ozone Hole? The Antarctic Ozone Hole refers to a severe depletion of ozone which has been observed to take place over Antarctica in springtime every year since the early 1980s. The standard definition is the region where the total column ozone is less than 220 Dobson Units (DU). It should be kept in mind that the word hole does not imply there is no ozone at all over a particular point. When did the Antarctic Ozone Hole first appear? In the early 1980s, the British Antarctic Survey noticed that springtime ozone values measured by their Dobson spectrometer at Halley since 1958 had been dropping since the 1970s. The Japanese station at Syowa also measured decreases in ozone.

In 1985, the decreases over Halley were reported in a now-famous article by Farman, Gardiner and Shanklin published in the journal Nature. Satellite observations then revealed that the depletion was actually taking place over a wide area across Antarctica, and the term ozone hole came into widespread use. How large is the Antarctic Ozone Hole? The maximum size of the Antarctic Ozone Hole reached each year increased rapidly from the early 1980s up until the early 1990s. Since then, however, the maximum area each year has almost always been in between 25 and 30 million square kilometres, the exceptions being the years 2002 and 2004 where it was somewhat less. (For comparison, the area of Australia is 7.7 million square kilometres). The largest areas so far observed have been 30 million square kilometres in 2000 and 29 in 2006. The years 1998, 2003 and 2005 also saw very large holes form. What causes the Antarctic Ozone Hole to form?

Three essential ingredients are necessary for the Antarctic Ozone Hole to form: ozone depleting substances, cold temperatures and sunlight. Ozone depleting substances were produced in industrial areas all over the world but over the course of many years, make their way into the stratosphere and are eventually transported to the polar regions. While the stratosphere is very dry, at very cold temperature (below -78C), clouds can nonetheless form, known as Polar Stratospheric Clouds (PSCs). PSCs form over a large area of Antarctica during winter. On the surface of these clouds, chemical reactions take place which convert chlorine and bromine into highly reactive forms, which in the presence of sunlight, undergo further reactions which are able to destroy vast numbers of ozone molecules. The Antarctic Ozone Hole thus appears when sunlight returns to Antarctica at the end of winter. Why does the Ozone Hole only occur over Antarctica? The chemical reactions that lead to the formation of the Antarctic Ozone Hole take place on the surface of Polar Stratospheric Clouds (PSCs), which can only form in very cold conditions (-78C).

Because of differences in the geography of the northern and southern hemispheres, the temperature over Antarctica in winter is somewhat colder than over the Arctic, resulting in PSCs being able to form over a much greater area and for a longer duration. This is the main reason why ozone depletion in the Arctic, while significant, does not result in a sustained ozone hole forming. Does the Antarctic Ozone Hole ever come over Australia? No. The ozone hole has only ever been observed to be well south of the Australian mainland and Tasmania. In fact, during springtime, when the hole is in existence, ozone levels over southern Australian cities are at their highest. However, after the ozone hole has broken up parcels of ozone depleted air mixed with mid latitude air move northwards. These parcels can move over the southern part of Australia and cause a reduction in total ozone values. How are ozone levels measured? Australia takes part in the Global Atmosphere Watch (GAW) international monitoring and research program coordinated by the World Meteorological Organization (WMO). The Bureau operates a network of ozone monitoring stations (at Melbourne, Brisbane, Darwin, and Macquarie Island), where the total amount of ozone above the station (total column ozone) is measured several times every day.

The total column ozone is measured with an instrument called a Dobson spectrophotometer, which compares the amount of sunlight at two ultraviolet wavelengths, one wavelength being strongly affected by ozone and the other one not. Total column ozone can then be calculated from the difference in the sunlight measured at the two wavelengths. The Bureau also flies ozonesondes weekly from Macquarie Island and Melbourne (using the Bureaus training annexe at Broadmeadows), as well as a partial program at Davis in Antarctica funded in part by the Australian Antarctic Division. Ozonesondes are devices which measure the ozone concentration in air electro-chemically. Carried into the air by a hydrogen balloon, they are then able to determine the ozone profile at high resolution from the ground up to a height of as great as 35 kilometres. Since the late 1970s the ground-based networks have been supplemented by satellite instruments which are able to measure total ozone and to a certain extent, ozone profile, at high temporal and spatial resolution. The global ground-based network is still essential however to provide a reliable and well-calibrated long-term record. What is a Dobson unit? Ozone is measured in Dobson Units (DU).

300 DU is equivalent to a 3 millimetre thick layer of pure ozone at sea level temperature and pressure. The Dobson Unit is named after G.M.B. Dobson, the English physicist who pioneered the study of stratospheric ozone. What is the Montreal Protocol? After the discovery of the Antarctic Ozone Hole, governments around the world moved quickly to adopt the Vienna Convention for the Protection of the Ozone Layer in March 1985. Work then began on negotiating a protocol to control the production and use of ozone depleting substances across the world, leading to the adoption of the Montreal Protocol on 16th September 1987, now celebrated as The International Day for the Preservation of the Ozone Layer. Since then, a number of Amendments to the original agreement have also been adopted to hasten the phasing-out of ODSs and to increase the number of substances controlled by the Protocol. To date, the Montreal Protocol has been ratified by one hundred and ninety three countries. Has the Montreal Protocol been effective? Yes. By 2006 consumption of ODSs globally had been reduced by over 96% compared to 1986 levels.

Although the atmospheric lifetimes of many ODSs are very long (greater than a hundred years in some cases), the total concentration measured in the troposphere has now been declining since the mid 1990s. The World Meteorological Organization (WMO) puts out regular Antartica Ozone Bulletins . More information can be found from the following organisations: United Nations Environment Program Ozone Secretariat: Ultraviolet Radiation: How It Affects Life on Earth : Feature Articles Ozone is the combination of three oxygen atoms into a single molecule (O3). It is a gas produced naturally in the stratosphere where it strongly absorbs incoming UV radiation. But as stratospheric ozone decreases, UV radiation is allowed to pass through, and exposure at the Earths surface increases. Exposure to shorter wavelengths increases by a larger percentage than exposure to longer wavelengths. Scientists can accurately estimate the amount of UV-B radiation at the surface using global data from satellites such as NASAs TOMS (Total Ozone Mapping Spectrometer), GOME (Global Ozone Monitoring Experiment) and Aura (will open in a new window), to be launched in 2003, satellites.

These satellite measurements are compared to ground-based measurements to ensure that the satellite data are valid. To calculate the reduction of UV-B by ozone, scientists consider the total ozone in a column of air from the stratosphere to the Earths surface. At mid-latitudes, a decrease of one percent in ozone may result in an increase of between one (310 nm) and three (305 nm) percent of potentially harmful UV-B at the surface during mid-summer when UV-B is highest. Ozone depletion is greater at higher latitudes, (toward the North and South Poles) and negligible at lower latitudes (between 30 degrees N and 30 degrees S). This means that decreases in ozone over Toronto are likely to be greater than those over Boston, and those over Boston greater than those over Los Angeles, while Miami will typically see the least ozone depletion of the four cities. However, cities at lower latitudes generally receive more sunlight because they are nearer the equator, so UV levels are higher even in the absence of ozone depletion. If ozone were to decrease at lower latitudes, southern cities would experience a greater absolute increase in UV-B than cities in the north for the same amount of ozone depletion. The U.S.

Department of Agriculture maintains an extensive network of radiometers to monitor ultraviolet B (UV-B) radiation across the country. The one pictured above is in Beltsville, Maryland. (Photograph by Jeannie Allen) Oblique angle of sunlight reaching the surface At any given time, sunlight strikes most of the Earth at an oblique angle. In this way, the number of UV photons is spread over a wider surface area, lowering the amount of incoming radiation at any given spot, compared to its intensity when the sun is directly overhead. In addition, the amount of atmosphere crossed by sunlight is greater at oblique angles than when the sun is directly overhead. Thus, the light travels through more ozone before reaching the Earths surface, thereby increasing the amount of UV-B that is absorbed by molecules of ozone and reducing UV-B exposure at the surface. The three images above illustrate how a change in angle between the sun and the Earths surface affect the intensity of sunlight (and UV-B) on the surface. When the sun is directly overhead, forming a 90° angle with the surface, sunlight is spread over the minimum area. Also, the light only has to pass through the atmosphere directly above the surface.

An increased angle between the sun and the surfacedue to latitude, time of day, and seasonspreads the same amount of energy over a wider area, and the sunlight passes through more atmosphere, diffusing the light. Therefore, UV-B radiation is stronger at the equator than the poles, stronger at noon than evening, and stronger in summer than winter. (Illustration by Robert Simmon) Aerosols Unlike clouds, aerosols in the troposphere, such as dust and smoke, not only scatter but also absorb UV-B radiation. Usually the UV reduction by aerosols is only a few percent, but in regions of heavy smoke or dust, aerosol particles can absorb more than 50 percent of the radiation. While the presence of aerosols anywhere in the atmosphere will always scatter some UV radiation back to space, in some circumstances, aerosols can contribute to an increase in UV exposure at the surface. For example, over Antarctica, cold temperatures cause ice particles (Polar Stratospheric Clouds) to form in the stratosphere. The nuclei for these particles are thought to be sulfuric acid aerosol, possibly of volcanic origin.

The ice particles provide the surfaces that allow complex chemical reactions to take place in a manner than can deplete stratospheric ozone. The eruption of Mt. Pinatubo in 1991 injected sulfate aerosols into the stratosphere, significantly though temporarily depleting stratospheric ozone and resulting in an increase of UV-B reaching the Earths surface. Over millions of years, the biosphere has evolved to deal with temporary increases in UV from reductions in stratospheric ozone by natural causes such as volcanic eruptions, but has not had the time required to adjust to long-term ozone reductions attributed to human activities of the last 30 years. (Photograph courtesy USGS) Water Depth UV-B exposure decreases rapidly at increasing depths in the water column. In other words, water and the impurities in it strongly absorb and scatter incoming UV-B radiation. Some substances that are dissolved in water, such as organic carbon from nearby land, will also absorb UV-B radiation and enhance protection of microorganisms, plants, and animals from UV-B. Different masses of water at different locations contain different amounts of such dissolved substances and other particles, making evaluation of UV damage very difficult.

Ultraviolet B (UV-B) radiation reaches different depths in ocean water depending on water chemistry, the density of phytoplankton, and the presence of sediment and other particulates. The map above indicates the average depth UV-B penetrates into ocean water. At the depth indicated, only 10 percent of the UV-B radiation that was present at the waters surface remains. The rest was absorbed or scattered back towards the ocean surface. (Image courtesy Vasilkov et al., JGR-Oceans, 2001) Elevation Living organisms at high elevations are generally exposed to more solar radiation and with it, more UV-B than organisms at low elevations. This is because at high elevations UV-B radiation travels through less atmosphere before it reaches the ground, and so it has fewer chances of encountering radiation-absorbing aerosols or chemical substances (such as ozone and sulfur dioxide) than it does at lower elevations. Ecosystems at high altitudes, such as this lake in the Rocky Mountains of Colorado, receive more exposure to ultraviolet radiation than ecosystems at low altitudes.

(Photo courtesy Philip Greenspun © 1994) Reflectivity of the Earths Surface As a highly reflective substance, snow dramatically increases UV-B exposure near the Earths surface as it reflects most of the radiation back into the atmosphere, where it is then scattered back toward the surface by aerosols and air molecules. Fresh snow can reflect much as 94 percent of the incoming UV radiation. In contrast, snow-free lands typically reflect only 2-4 percent of UV and ocean surfaces reflect about 5-8 percent (Herman and Celarier 1997). Effective Communication at Pepsi Co - Term Paper Effective Communication at Pepsi Co Which Indian state is at the eastern end of the Himalayas? A: Assam. What is the name of the atmospheric gas which screens out the sun's harmful ultraviolet radiation? A: Ozone. What is the world's deepest ocean? A: Pacific. Which is the largest animal ever to have inhabited the Earth? A: Blue Whale. What once covered 14% of the Earth's land area, but by 1991 over half had been destroyed? A: Rainforest.

Which inland sea between Kazakhstan and Uzbekistan is fast disappearing because the rivers that feed it have been diverted and dammed? A: Aral Sea. The damaged Chernobyl nuclear power station is situated in which country? A: Ukraine. What type of rock is granite? A: Igneous. What type of rock is basalt? A: Igneous. What is the main constituent of natural gas? A: Methane. What is the term for nutrient enrichment of lakes? A: Eutrophication. Which of the Earth's atmospheric layers reflects radio waves? A: Ionosphere. Which gas forms 80% of Earth's atmosphere? A: Nitrogen. In which mountain chain would you find Mount Everest? A: Himalayas. What is the collective term for substances such as coal, oil and natural gas, the burning of which produces carbon dioxide? A: Fossil fuel. What contributes to the greenhouse effect at lower atmospheric levels, but in the upper atmosphere protects life on Earth? A: Ozone. What is the name of the process by which substances are washed out of the soil? A: Leaching. Who was director of the environmental pressure group Friends of the Earth 1984 - 90?

A: Jonathon Porritt. Which European country is committed to decommissioning all of its nuclear reactors? A: Sweden. Which Canadian city gave its name to the 1987world agreement on protection of the ozone layer? A: Montreal. Five-legged creatures have damaged which 1250 mile long wonder of the world? A: Great Barrier Reef. Ultraviolet Radiation Plexus-NSD What is ultraviolet radiation? Ultraviolet radiation, abbreviated as “UV radiation”, is a form of non-ionizing radiation. Before you go on, I’m not sure I really understand the difference between ionizing and non-ionizing radiation. Can you give me a hand here? Sure. To make a long story short, non-ionizing radiation does not have enough energy to ionize atoms. However, it can cause molecules to vibrate and rotate, which means things will start heating up. Typically, the different types of non-ionizing radiation are classified by frequency, which is stated in units of “hertz”, abbreviated as “Hz”. What are these classifications? These include ultraviolet radiation, visible light, infrared radiation (abbreviated as “IR”), radio- frequency (abbreviated as “RF”) and microwave radiation, and ultrasound.

In addition, lasers use many of these frequencies to form coherent light. All of these are electromagnetic radiations. What’s the difference between ultraviolet radiation and the other forms of non-ionizing radiation? The region of the electromagnetic spectrum just below that of visible light is called the “near ultraviolet” region. This type of radiation is absorbed (blocked) very strongly by most solid substances, and even absorbed appreciably in air. The shorter UV wavelengths can reach the ionization energy for certain types of molecules, so that region, termed the “far ultraviolet” region, may trigger similar considerations as other ionizing radiations. So what does UV radiation do? Generally speaking, and depending upon the amount of UV radiation that we are exposed to, it can burn the skin. However, there are also some therapeutic effects as well. Is ultraviolet radiation able to penetrate solid surfaces like ionizing radiation? Well, near ultraviolet radiation is absorbed very strongly in the surface layer of the skin by electron transitions. But you need to have much higher energies in order to trigger the photoionization process. When this occurs, there can indeed be penetration into surfaces. I understand that ozone can protect us from the harmful effects of UV radiation. Is that correct? Absolutely.

The ozone layer in the upper atmosphere is important for human health because it absorbs most of the harmful ultraviolet radiation from the sun before it reaches the surface. However, the higher frequencies in the ultraviolet region can still pass through the ozone layer and produce harmful physiological effects ranging from sunburn to skin cancer. Was ultraviolet radiation discovered at the same time as forms of ionizing radiation, like x- rays? No. Scientists were measuring the effects of ultraviolet radiation long before they discovered x- rays and gamma rays. How far back? Ionizing radiation was discovered in the late 1800s by scientists like Roentgen and Curie. However, as far back as 1666 Sir Isaac Newton allowed a sunbeam to enter into a darkened room through a small hole, pass through a prism, and fall upon the opposite wall. What appeared was a spectrum of very vivid and intense colors. By means of further experiments he was able to prove that white light was made up of several different components. But that wasn’t UV light, was it? No. However, in 1800, Sir William Hershel discovered that the solar spectrum extended out beyond the portion which was visible to the human eye.

He formed a spectrum much like Sir Isaac did, then passed a thermometer through the various colors. He then compared the reading with that of a similar thermometer shaded from the spectrum. What he found was a temperature rise as he passed from the violet to the red end. Still more astounding was the fact that the thermometer showed a higher temperature in the dark space beyond the red region of the spectrum than it did in any part of the visible spectrum. What did this mean? This meant that there was some intense radiation beyond the region which was visible to the eye. This invisible radiation beyond the red end of the spectrum is now known as infrared radiation. Interesting. So is that how UV radiation was first classified? No. It wasn’t until 1801 that a scientist known as J.W. Ritter investigated the other end of the spectrum. He showed that chemical action was caused by some form of energy in the dark portion beyond the violet. At that point in time, the region became known as the “ultraviolet” region. Tell me where I can find ultraviolet radiation. There are both natural and artificial sources of UV radiation. Although there are many artificial sources of this energy, sunlight is the only natural source. Tell me a bit more about sunlight.

The sun emits a wide variety of electromagnetic radiation, including infrared, visible, and what is termed “ultraviolet A” radiation (UVA; 320 to 400 nm), “ultraviolet B” radiation (UVB; 290 to 320 nm), and “ultraviolet C” radiation (UVC; 10 to 290 nm). Blue visible light lies in the range of about 400-500 nm. The only UV wavelengths that reach the Earth’s surface are UVA and UVB, with UVA radiation being 1,000-fold less effective than UVB in producing skin redness. However, its predominance in the solar energy reaching the Earth’s surface (tenfold to one hundredfold more than UVB) permits UVA to play a far more important role in contributing to the harmful effects of sun exposure than previously suspected. Can I run into UVB at any time? Yes, whenever the sun is out. However, the intensity of UVB is typically at its highest when the sun is directly overhead (i.e., between 10:00 a.m. and 2:00 p.m.).

On the other hand, UV-B is attenuated (reduced) by the ozone layer to the extent that reductions of a factor of 10 at 9:00 a.m. and 3:00 p.m. are observed, relative to the intensity at noon. Does all UV radiation affect the body in the same manner, regardless of wavelength? The UV light spectrum can be subdivided into three subsections, based upon the biological effects identified within each. These are the regions I told you about previously, meaning the UVA, the UVB and the UVC regions. What will the UVA region do? The UVA region is more commonly known as the “black light region”. You have probably run into this kind of radiation in amusement park rides where UVA is used to enhance visual thrills. There are no real health effects associated with exposure to UVA. What about UVB radiation? Light in the UVB region will cause skin to tan. However, if you’re not careful, it can also cause serious sun burns. And UVC radiation? The UVC region is sometimes called the “germicidal region”. This is because UVC light is sometimes used for sterilization.

So, which of these wavelengths can I see with my eyes? All UV radiation, regardless of its wavelength, is invisible to the eye. The only way to measure the intensity of UV radiation is to use a calibrated survey instrument. You said that UV is created by the sun. Is the intensity of radiation the same from one city to the next? Sunlight is definitely the greatest source of human UV radiation exposure, affecting pretty much everyone. The extent of an individual’s exposure, however, varies widely depending on a multiplicity of factors such as clothing, occupation, lifestyle, age, and geographic factors such as altitude and latitude. There is greater UV exposure with decreasing latitude. Residing at higher altitude results in a greater UV exposure such that for every 1,000 feet above sea level, there is a compounded four (4) percent increase in UV exposure. However, as I said earlier, UV exposure increases with decreased stratospheric ozone. What else influences the magnitude of UV exposure? A number of things. Some examples are heat, wind, humidity, pollutants, cloud cover, snow, season, and time of day. Let me go back to ozone for a second. It seems that I hear a lot in the news about the depleted ozone layer.

Is ozone really necessary to protect people on earth? If solar radiation reached the surface of the earth unscreened, it would be lethal to exposed living organisms. In practice, the shielding provided by the atmosphere results in substantial attenuation of the UV light, with ozone providing a major contribution. Does the amount of ultraviolet radiation stay the same each day? No. Solar flares, known as “sunspots”, may change the amount of UV radiation reaching the earth on any given day or from one season to another. Solar flares increase ozone concentration in the stratosphere (above 50 km) thereby reducing the amount of surface UVB. When solar flares are inactive, there is a decrease in the ozone concentration, allowing increased UVB to penetrate to the Earth’s surface. Ozone is the main defense to manage our everyday exposure to ultraviolet radiation. Changes in the concentration of the ozone in the atmosphere may change the intensity of ultraviolet radiation on the surface of the earth. You said there were some artificial sources of ultraviolet radiation, right? Yes I did. There are many artificial sources which may result in an exposure at work or at home.

some examples are those used for medical therapy or a tanning booth, or tungsten halogen lamps, and even arcs from welding operations. However, there are few artificial sources that result in human exposure greater than from solar radiation. You also said they use ultraviolet radiation to sterilize things and kill bacteria. How does that work? Disinfection of water generally uses low pressure mercury vapor lamps to generate ultraviolet radiation. These lamps generate short wave ultraviolet in the electromagnetic spectrum region of about 254 nanometers. Light in this region is lethal to micro-organisms including bacteria, protozoa, viruses, molds, yeasts, nematode eggs and algae. How much ultraviolet radiation does it take to kill bacteria? Ultraviolet dosage is measured in “microwatt seconds per square centimeter”. In essence those units mean that the higher the microwatt level, the higher the dosage. Or the longer the exposure time, the higher the dosage. Finally, the greater the exposed area, the higher the dosage. For your information, the U.S. Public Health Service published a policy stating that the amount of UV necessary to sterilize drinking water is at least 16,000 microwatt seconds per square centimeter.

I thought most drinking water was sterilized with chlorine. Why use ultraviolet radiation? You’re right. There are indeed two common methods of disinfecting water: chlorine and ultraviolet (UV) treatment. Both have long been used to destroy bacteria. However, UV is very effective in killing virtually all disease-causing viruses and bacteria, and it does it in a very few seconds. What about chlorine? Chlorine, in the past, was the sterilizing method of choice, and was more widely accepted than UV. Lately, however, chlorine has been strongly criticized because it not only affects the taste and odor of the water it disinfects, chlorine also produces cancer-causing by-products in some waters. It has also become a much more expensive treatment option than UV. So, we now see many more water treatment facilities converting their treatment operations over to the use of UV to kill bacteria and mold. What about light bulbs? I heard somewhere that flourescent lights use ultraviolet radiation to make visible light. I thought you told me that ultraviolet light is invisible to the eye. A flourescent light fixture uses ultraviolet radiation to “create” visible light. How do they do that?

Electronic circuits in the base of the bulb energize a magnetic coil, creating a magnetic field. The field then activates the mercury gas in the bulb, which emits ultraviolet radiation. Visible light is released as the ultraviolet light passes through a phosphor coating on the inside surface of the bulb. The color of the visible light can be changed by changing the phosphor coating and the makeup and concentration of the gas inside the tube. What kind of bulbs do they use in a tanning booth? Are those flourescent lights? Yes they are. Fluorescent lamps can be designed for special applications like tanning booths. These emit levels of UV sufficient to cause injury to the skin and eye. The most potent artificial sources of UV, and particularly of UVB and UVC, are those characterized as high intensity discharge (HID) lamps. These include high pressure mercury, mercury metal halide and neon lamps. High pressure mercury vapor lamps with little or no filtration for ultraviolet radiation were once commonly used for cosmetic tanning. These lamps are now little used, having been largely replaced by UVA fluorescent and filtered HID UVA cosmetic tanning systems. Are the tanning booths safe to use? Is there too much ultraviolet radiation?

The tanning industry is rapidly growing in the United States. Currently, more than a million Americans use commercial tanning facilities every day. The biggest categories of users are adolescents and young adults, especially women. While short (less than 20 minutes) exposure to UV radiation may, indeed be beneficial to health since it causes the exposed skin to generate vitamin D, overexposure may result in the burning of exposed skin and serious eye effects. Eye effects? Yes. Eye exposure is especially dangerous because the results of over exposure are not immediately evident. There is a potential for skin cancer with constant overexposure. I’ll tell you more about that in just a minute. Any other common sources of ultraviolet radiation? Gas welding, brazing and cutting processes operate at temperatures insufficiently high to cause the emission of intense UV radiation. Arc welding processes are particularly potent sources of UV radiation and even very short exposures may be hazardous to the eyes and skin. Both gas and arc welding also emit visible and infrared radiations which may be hazardous to the retina. Appropriate personal protection always needs to be worn during welding. Let’s go back to effects of UV radiation. Can you tell me more about them? UV interacts with tissue predominantly by photochemical effects.

Those organs that receive the greatest damage are the skin, the immune system, and the eyes. The extent of interaction with the skin depends on the wavelength, skin pigmentation, and tissue thickness. Typically, the shorter UVB wavelengths are absorbed by the outer layers of skin and, as the wavelength increases, the depth of penetration increases. Short-term effects on the skin may be seen as sunburn, principally consisting of erythema (skin reddening resulting from expansion of blood vessels) and edema (swelling), both of which may be very severe. In some people this sunburn is followed by increased production of melanin and is recognized as a suntan. So that’s it. All I need is a good tan and I’m set. Well, it is not that simple. A suntan is not an indication of good health and only offers minimal protection against further exposure. It is a sign that damaged skin is attempting to protect itself from further harm. I can tolerate a sunburn every now and then. What’s the problem? The most serious health effects, for which exposure to UV radiation is a recognized risk factor, are the skin cancers.

UVB has been recognized for some time as carcinogenic in experimental animals, and there is increasing evidence that UVA, which penetrates more deeply into the skin, also contributes to the induction of cancer. Basal cell cancer, the most common skin cancers in Caucasians, are found primarily on sun-exposed areas such as the head and neck, and hands. People with light complexion and who sunburn easily have a much greater likelihood of tumors. Are there lots of UV-induced skin cancers? Yes. Skin cancers in which UV radiation exposure plays an important role are now the most common form of cancer. In 1978, there were more than 500,000 new cases of UV-induced skin cancer. Does skin cancer spread throughout the body like other soft tissue tumors? No. The non-melanoma skin cancers are mainly basal cell carcinomas and squamous cell carcinomas. They are relatively common in Caucasian populations, although they are rarely fatal. Unfortunately, the incidence of skin cancer is increasing year after year. As I said before, they occur most frequently on sun-exposed areas of the body such as the face and hands and show an increasing incidence with increasing age.

Does every person have the same chance of getting skin cancer from a sunburn? No. Some people are more likely to have skin cancer than others. For example, people reporting skin cancers have light skin (melanin absorbs UV radiation), red or blond hair, blue eyes, or a tendency to burn and not tan on sun exposure. There is extensive epidemiological evidence supporting the direct role sunlight plays in human skin cancer. How do I know if I have skin cancer? The major signs of suspected malignant melanoma are: an existing mole getting larger, developing an irregular outline, or showing shades of brown and black; or a new mole growing quickly (in months). Minor signs include: a mole becoming bigger than the blunt end of a pencil (around 5 mm), or becoming inflamed or developing a reddish edge, or bleeding, oozing or crusting; or a mole starting to feel different (e.g. itching or painful). If you have any questions or suspicions, contact your physician for specific information. Can UV damage other parts of the body? Absolutely. Too much UV may damage your eyes; photokeratitis and photoonjunctivitis (inflammation of the cornea and the conjunctiva, respectively).

The damage is most likely reversible but painful for several days. Are there rules, laws or standards designed to limit exposures to UV or eliminate damage to the skin and eyes? There is no OSHA standard for UV radiation exposure. However, the National Institute of Occupational Safety and Health (NIOSH) recommends exposures to the skin or eyes be less than 1,000 microwatt per square centimeter for periods of greater than 1000 seconds. This NIOSH limit, however, changes depending on the specific frequency that is present. Are there any simple measures that I can take now to reduce my chances of skin damage? Yes. First, damage can be minimized by using clothing made of tightly woven fabrics with long sleeves, long pants, wide-brimmed hats, etc. Okay, I can do that. Anything else? Yes. Sun screen and certain suntan lotions are useful. Minimum protection is afforded by chemical sun screens with protection factor ratings of 15 or higher. Although most sun screens on the market today are appropriate for UVB protection, combination sun screens that are effective against UVB and at least part of the UVA spectrum are preferable.

Waterproof sun screens should be selected by swimmers and those who perspire sufficiently to wash off non- waterproof products. I can do that too. Anything else? Most importantly, limit the amount of time that you are exposed. Research data suggest that 50 percent of an individual’s total lifetime UV radiation exposure occurs by 18 years of age. Parents can play an important role in protecting their children by managing outdoor activities at school, camp, daycare centers, or the beach. Remember that more than 60 percent of the daily UVB radiation reaching the Earth’s surface arrives during a sunny day in June between 10:00 a.m. and 3:00 p.m. Where can I get more information about exposures to ultraviolet radiation? The “Links” section of the Plexus-NSD web page is a good place to start. If you go to the category of “Nonionizing Radiation”, you will see several references to UV light and UV exposure. Any final words? Yes, just a final note on the issue of UV exposure and skin cancer.

Because so many of these cancers can be prevented by reducing exposure to UV radiation, the “Health Physics Society”, a distinguished professional organization, has issued a position statement that advises the public, health officials and the news media on ways of reducing the risk of UV-induced cancer. What are the Society’s recommendations? Pretty much the same as I told you about earlier. However, they bear repeating. First, they advise people to avoid the use of tanning beds and sun lamps. In fact, they state that unless directed by a physician, people should not use this equipment because it offers very little health benefit but significantly increases the risk of cancer. Do they also offer protection recommendations? Yes they do. To protect yourself from the sun, the Society advises you to minimize exposure when the sun’s rays are the strongest. (They even tell you that if your shadow is shorter than you are, you should seek the shade.) You are also advised to apply a broad-spectrum sun screen that protects against both UVA and UVB, and that has an SPF of at least 15. Finally, they tell you to reapply sun screen every two hours, even on cloudy days, to wear a wide-brimmed hat and sunglasses, and to avoid reflective surfaces.

Harmful radiation harmful radiation ECO-PROS Third from the Sun Coral reefs are being damaged and dying from harmful human activities. The deteriorating condition of coral reefs is causing great environmental concern. Protection ... a very thin layer of atmosphere provides our life support system and protects us from harmful radiation. Approximately 110,000 million tons of carbon dioxide is released into the atmosphere each ... ECO-PROS - Nature Knows Best To provide a blanket of warmth around the planet The atmosphere is being damaged by carbon dioxide overload, so more harmful radiation is reaching us ... AIR, WATERS AND SOIL CLEAR-CUTTING FORESTS DUMPING TOXIC WASTE CUTTING OLD-GROWTH TREES HARMFUL FISHING METHODS DEPLETING AND WASTING NATURAL RESOURCES DESTRUCTION OF OZONE LAYER DESTRUCTION OF BIOLOGICAL ... (UV) rays from the sun, protecting the Earth from this harmful radiation. But down here in the troposphere, where we all live and breathe, ozone turns from ... The Montreal Protocol ... EPA Earth's sunscreen - the ozone layer The ozone layer screens out the suns harmful ultraviolet radiation. Can we reverse its destruction?

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Question: What is the name of the atmospheric gas which screens out the sun's harmful ultraviolet radiation?
Answer: Ozone